I got your point. I was being intentionally brief.
I’ll clarify:
1) There are linguistic rules to LHK – phonological, morphological and syntactic. They exist whether or not reading/pronunciation changed due to surrounding languages. The most basic rules can pretty much be derived from any other Semitic langauge that was wasn’t essentially a liturgical language for nearly 1900 years (which is what Hebrew for the most part was, from Churban Bayis Sheni until Ben-Eliezer).
2) Teimanim (especially Baladi) appear to have the most uncorrupted reading tradition (mainly due to their relative isolation). If we want a vague idea of what LKH is supposed to sound like, look at the Baladim. Speaking of Teimanim in general, HaKatan is 100% wrong about shmad – Teimanim and Sephardim absolutely use the “th” sounds in davening and leining (anyone who has davened at a Sephardic shul or learned in a predominantly Sephardic Hesder Yeshiva (like I have) would know this – Sephardim are far more precise with pronunciation in their leining and davening than Ashkenazim are and they make a point of being so (the exception to Ashkenazim being less precise is obviously those whose tradition is נוסח אשכנז מובהק).
3) The varying pronunciations all have errors and that’s because a variety of cultural factors, time + מנהג אבותינו בידינו (sociolinguistic factors). For Teimanim, it’s ג being read as /j/; for Chassidim, pretty much everything, but especially vowels (there is at least one Chassidish Rebbe (IIRC Vizhnitz), who said he’s never heard a 100% kosher Torah reading in his life); for Litvaks, it’s cholam, as it is for Galitzianers. Ashkenazim don’t differentiate between א and ע, ח and כ, ט and ת, which they should. For Sephardim in general, their vowels are from Arabic, not Hebrew.
4) Incorrectly reading words is a problem על פי הלכה and we know this. Again, I refer to מסכת ברכות and statements about not allowing those who cannot differentiate between א and ע to serve as a שליח ציבור.
5) We have our traditions and they aren’t going away and I never said only one tradition is correct. The differences (acutally errors) in pronucnciation obviously aren’t going anywhere. But that does not change the fact that there are linguistic rules and they should be followed (and those who בשיטה don’t should obviously go back to reading correctly).
6) I never addressed Modern Hebrew. I tend to follow the opinion of Rav Kook זצ״ל that Modern Hebrew is 100% פסול insofar as fulfilling a חיוב and that Ashkenazim should be reading per Ashkenazi tradition only (and the other traditions per their Mesorah only as well, obviously). Which is why, despite having lived in Israel for 15 years (and B”H having very good Hebrew), I still daven and lein in the Ashkenazi tradition I grew up with. But I’m a distinct minority.
7) My last statement was directed at the OP, not you, asking why he cares what Yiddish does with regard to its Hebrew component. Yiddish isn’t a Semitic language, so there is no butchering of Hebrew to be done.